Inertial navigation systems are basically a triad of accelerometers in combination with a triad of gyroscopes which provide acceleration information in a known coordinate frame to a navigational computer. The computer integrates the accelerations in an appropriate navigation reference frame from appropriate initial conditions to provide continuous measures of velocity, position and instrument frame attitude.
The accelerometers measure specific acceleration made up of platform acceleration in linear combination with gravity but since gravity is not perfectly known, an error may develop. This error together with Inertial Navigation System (INS) instrument errors result in navigation solution errors. These navigation errors grow unbounded predominately at or near zero, schuler and siderial frequencies.
The INS vertical position solution is dynamically unstable since the computer must use vertical position and latitude to compute and compensate measured accelerations for normal gravity. Negative feedback results so that vertical position diverges exponentially.
Conventional INS practice is to integrate a height sensor (depth gauge, altimeter, surface ship sea level knowledge) with the INS. The difference between inertial vertical position and that of the height/depth sensor is used to stabilize the inherently unstable vertical channel.
Although the prior art which integrates a conventional three axis strapdown, partially or fully stabilized INS with a height sensor improves navigation performance, it does not fully exploit navigation system velocity error observability. If this velocity error observability (which is significantly enhanced if a gravity field anomaly map is also integrated) is exploited, these errors can be bounded. The anomalous gravity field map can also be used in a map matching mode to bound position error as well.